Answer:
The apparent change in frequency of sound is explained by the Doppler effect of sound.
The change occurs due to the change in the relative velocity of the observer and source of sound, due to their relative motion.
Explanation:
When an object is stationary and listening to a stationary source of sound, like a factory alarm, the frequency of sound heard by the observer is equal (same) as the frequency given out by the source of sound.
When the observer stays at rest but a car blowing horn approaches him, the frequency of sound heard increases. The apparent frequency is higher than the original sound coming out from the car horn.
If the car is speeding away from the observer, apparent frequency heard by the stationary observer decreases.
Like this, several other conditions can be explained based on the relative motion of observer and source of sound.
